This invention relates in general to firearms and, in particular, to a new and useful apparatus for braking a free piston driven at a high acceleration within a tube, by plastic deformation of material, comprising a deformable braking sleeve which is fixed to the tube wall in the zone of that tube end toward which the piston is accelerated, in particular, a braking sleeve, having its end facing the piston beveled toward the inside to a knife-shaped edge and deforming, upon the impingement of the piston and starting from this edge, radially inwardly.